The “vascular unloading technique” or “volume-clamp method” introduced by Peñaz in the early 1970's provided for a new method to continuously and non-invasively monitor arterial blood pressure (CNAP). Devices exist that include a band that is externally placed over adjacent fingers containing arteries of comparable size. A sensor detects pulsating changes in the arteries based on the amount of light transmitted through the tissue. The transmitted light intensity is used to measure the blood volume under the finger artery. The pressure in the cuff is changed by compression and decompression to keep the blood volume substantially constant. By maintaining a constant blood volume, the arterial wall is relaxed and the cuff pressure is substantially identical to the pressure in the underlying artery. The cuff pressure therefore reflects the intra-arterial pressure under the finger cuff and continuous non-invasive beat-to-beat pressures can be calculated.
In addition to continuous non-invasive blood pressure measurements, obtaining information regarding the blood oxygen saturation level of a patient using a non-invasive monitoring mechanism is desirable in both diagnosis and treatment of medical conditions. Blood oxygen saturation level may be determined using a technique known as transmission spectrophotometry, or more widely known as pulse oximetry (SpO2). Conventionally, pulse oximetry measurements are obtained via a sensor positioned on a finger of the hand having the non-invasive blood pressure monitor connected thereto or on a finger on the contra-lateral hand. A drawback associated with this configuration is increased discomfort to the patient and an increase in the number of sensors connected to the patient making monitoring more costly and cable management more difficult for healthcare professionals. A further drawback is the increased chance that one or more sensors would become dislodged thereby disrupting patient monitoring.
Accordingly, there is a need for a device and a method that permits simultaneous CNAP and SpO2 measurements without increasing the number of separate sensors attached to a patient. In addition, it would also be desirable to provide a cuff design adaptable to the patient's finger circumference, thereby reducing measurement errors related to cuff application and cuff shifting on the finger. A system according to invention principles addresses deficiencies of known systems.